I've never had this happen to me before. But recently, some enemies have literally infinite hp. I first noticed it when trying to kill a guy that I could've easily killed with a few hits, but he never died.

Now I'm stuck on a boss. Ironically, it's Draygon that I can't get past. I've wasted 3 magic rings, which gets probably around 15 fully-charged level 3 thunder blasts. He should probably be dying in around 7.